As Annette pointed out; you cannot "take over payments" ; because the house is worth much less than the total of the payments, but the Bank is owed that much.
People have been trying to find shortcuts or ways around the system, for several years now; it doesn't exist; because the missings money, the difference, has to come from somewhere.
If a Buyer wanted to pay the difference in CASH and just assume the debt; aside from being stupid, (the buyer would be paying much more than the the house was worth,) the Appraisal would have to be less than 20% of the Value.
The Appraisal keeps the Buyer from paying too much.
Talk to a Realtor about helping you: The Buyer's Agent is paid from the Escrow and costs you nothing.